Posted
by
samzenpus
on Sunday October 03, 2010 @07:26PM
from the we're-going-to-need-more-ram dept.

astroengine writes "The scales are mind-boggling and the physics is cutting edge, so how do you go about simulating the collision of two galactic clusters? Using some of the most powerful computers in the world, researchers at Argonne National Laboratory, the Flash Center at the University of Chicago and the Harvard-Smithsonian Center for Astrophysics have done just that."

No. Sound is the vibration of air molecules, so when you speak or drop something, it creates compression waves that travel through the air and vibrate your eardrum, which in turn creates waves in the fluid of your cochlea that stimulate hair cells connected to the acoustic nerve. Since outer space has (almost) no air, these waves have no medium on which to travel, and sound as we know it does not happen.

Except x-rays and gamma rays are light, being on the electromagnetic spectrum, and traveling at 299,792,458 m/s. Sound only travels around 340 m/s, depending on its medium. Also, if they were sound they wouldn't travel through space either.

No. Sound is the vibration of air molecules, so when you speak or drop something, it creates compression waves that travel through the air and vibrate your eardrum, which in turn creates waves in the fluid of your cochlea that stimulate hair cells connected to the acoustic nerve. Since outer space has (almost) no air, these waves have no medium on which to travel, and sound as we know it does not happen.

Well, yes and no. There's no sound in space that a human could hear -- especially over the deafening roar of their blood boiling in the near-vacuum of space -- but there is a tremendous amount of diffuse gas and dust in galaxies and galaxy clusters, through which compression waves travel, albeit very weakly and slowly. If you were to observe those waves, then you could convert that data into an audio waveform in the range of human hearing. I may be misremembering, but I seem to recall that a group of researchers did precisely that with the (vastly smaller, nearer, and more easily observable) waves of gas being propelled outward by the pulsar at the heart of the Crab Nebula.

And yes, I know that really stretches the human notion of sound, but objects the size of galaxy clusters stretch most of our petty human notions, so it only seems fair.

I'd guess the opposite, as interactions between ions in plasma are a lot easier (delivered through electromagnetic force caused by the electric charge) than interactions between atoms/molecules in neutral gas (delivered through collisions).

However, on such a scale as this, who is to say this isn't similar to two clouds colliding? I would hazard a guess that on some microscopic level there is definitely a sound of water droplets colliding when two clouds merge. On a universal scale, I'd say two galaxies colliding would be quite close, proportionally, to that sound.

Actually, galaxy collisions are thought to leave solar systems undisturbed, with only a handful of collisions. The reason is that the space between the stars are so large compared to their size. Gas merges and spiral arms are distorted, but a planet would be fine. This is also what is expected of the Andromeda galaxy merge.

what does "handful" mean, in terms of percentage of solar systems?for instance, I understand that there are a lot of pluto sized objects orbiting our solar system. how close would a star (from another galaxy, which means it's moving in a completely different direction than the local surrounding stars) have to get to disturb those objects? I realize that in order to disturb a solar system, you need to disturb planet orbits (or destroy planets), but in order to seriously harm a civilization, considerably less

A solar system or planet might be "fine" in the astrological sense, but there is likely a much higher chance of objects in Oort clouds getting disturbed and sent hurtling towards your friendly neighborhood M class planet.

I used to do simulations of galaxy collisions, and dark matter is the easiest part to put into the simulation because it is assumed to be collisionless. It just goes wherever the gravitational potential points it (of course, calculating that gravitational potential can be somewhat complicated if you want to smooth over the rough spots caused by the finite number of particles simulated). The hard part is the gas, which has much more complicated physics due to the collisions, heating, cooling, and radiation

The Department of Energy totally loves this kind of publicity. They want you looking at their pretty pictures and saying "Oh, wow!" The bureaucrat with whom I constantly spar over this science (and computer budgets) by pictures denies that the pictures are all that important, but we are constantly bombarded by them. The science and the numerics here may be great or they may be garbage. It doesn't matter all that much, I claim, because the people who vote on budgets and write the checks would never know

Graphics give understanding, though. A numeric analysis can show exactly what happens, but it doesn't convey a general idea of what's going on. Pictures are easier to understand, and show more information at once. There's a reason why the weatherman shows his forecasts on a giant map.

Speaking of giant maps, I visited the ANL recently, and saw a computer system being used for related research. If they're using the same visualization system (which looks REALLY similar to the video in TFA), then this graphical model could be shown on a giant screen, and the model could be rotated & zoomed at any point. It's science through pictures, not just pictures of science.

The DoE recently published a "prediction" that showed the oil from Deepwater Horizon racing up the east coast. As far as I'm concerned, the assumptions and methodology of the simulation were indefensible, but that didn't stop the Wall Street Journal from picking it up. While the DoE's *official* position was that the simulation wasn't necessarily credible as a "prediction," one of the NCAR scientists involved was quoted as saying that they realized that they had "the perfect model" for predicting the fate

That's because the Wall Street Journal, like so many others, confuses the meaning of the visualizations. They aren't results. Instead, they're great tools for finding what parts of the theory need a better test.

As a contrived example, let's say that this visualization shows that a plume of dark matter going in a particular direction at a particular time. Comparing the visualization at that time to known colliding clusters in the real world might help show where to point our telescopes for evidence of dark matter. It helps to create the initial hypothesis, reducing the number (and therefore the cost) of failed experiments.

Another use is for verification of a model. If we already know of several colliding clusters, this visualization should, be able to produce images that look very similar to those clusters. If not, then we know that there's something wrong with the model, and we can find ways to improve it.

Tying that in with your example, we now know that the fluid model used wasn't perfect. It's time for more analysis, experiments, and refinements, eventually resulting in a more thorough knowledge of our universe.

No scientist worth their salt will say that any model is absolutely perfect. In fact, the one you spoke of didn't [intel.com]. She said it was the "perfect model to do <a given job>," implying that it could do the job with the given parameters, and that deriving a completely new model wasn't necessary. The model itself is imperfect, but it fit the job perfectly. If the journalists presented the model as a prediction, that's the journalists' fault.

You're talking to someone who has done a fair bit of simulation and listened to others describe their own simulations. The simulation the Wall Street Journal bought into was wildly beyond naive. I told the author to ask an oceanographer, and he did. A later article, without simulation pictures, gave a much more accurate assessment of the unknowns. I'm not an oceanographer, but I've done ocean simulation, and I'm very familiar with the computational techniques employed. As a predictive tool for that sit

Knowing something about science, unfortunately, is not enough. The problem with pretty pictures is that they are far too easy to make, and the level of detail offered by a typical fluid mechanical simulation implies something about the accuracy of the calculation that is often unsupportable in theory or in practice. What's the point of gorgeous pictures if the prediction is incorrect? We don't really know all that much about the detailed physics of hurricanes. Predictions of hurricane tracks are notorio

Scientists should be very careful about creating misimpressions of how well a problem is understood by misuse of color graphics. In the past, you would have gotten an "artist's conception." Now we get "simulations." The difference to which the actual physics are understood or accurately represented may not be all that great. When people discover, as some must have in the case of the Gulf oil spill, that what they're been fed is useless or worse, they rightfully become skeptical of the scientific enterpr

I'm confused by "As the two clouds of dark matter inside each cluster can only interact gravitationally"

If dark matter can interact gravitationally wouldn't this mysterious crap just accumulate in the gravity wells of massive objects like stars or even the earth the same way planets collect rocks and dust around them?

Especially since everyone seems to be saying that dark matter so outnumbers normal stuff wouldn't a significant portion of the total mass that contributes to gravity of our own sun and earth be from dark matter?

I don't doubt that dark matter contributes to gravity but to say that it has an effect in a way that would suggest it have "mass" is one of those moments where I go searching to find out what I don't understand this time because that makes no sense. If dark matter acted as "stuff" that had mass then surely it would clump!!

Photons are massless but they have energy and therefore contribute to the gravity field even though they are not effected by gravity in the same way a massive object would be...the only effect is travling thru the pit created in the metric by the presence of "stuff".

Please if there is anyone who can help me make sense of this I would be eternally grateful.

If dark matter acted as "stuff" that had mass then surely it would clump!!

I don't know about the scale of stars and planets, but it is strongly believed that DM "clumps" in and around galaxies. The effect on the rotation curves was one of the first things that tipped off the probable existence of such stuff.

I have similar ideas about dark matter. To me, DM is simply a way for scientists to explain their way out of issues we do not really understand. It's matter, but can not be detected. Doesn't interact with anything, apparently not even itself (in this simulation those "DM cores" simply pass through one another!), other than that it works on gravity. It's like aether - we don't know how it works, so we make something up to make it work. That's how it feels to me.

The same is true for gravity, science does not know what it is just describes how it behaves. Essentially ALL the fundemental forces and dimentions in the universe are "miricales" that have no explaination.

BTW the oldest star is thought to be 13.2byo, the universe is 13.7byo, meaning the first stars formed some 0.5 billion years after the big bang.

"If dark matter can interact gravitationally wouldn't this mysterious crap just accumulate in the gravity wells of massive objects like stars or even the earth the same way planets collect rocks and dust around them?"

Other way around. Normal matter (the minority) tends to clump in areas where there's lots of dark matter. Dark matter itself clumps, as you predict it would, but not to the extent that normal matter does. Normal matter clumps partly due to gravity, but to make nice tight clumps like stars yo